1. Field of the Invention
The present invention relates to a method for manufacturing biaxially oriented tube from thermoplastic material, comprising forcing the tube over a mandrel at an orientation temperature of the plastic material, which mandrel comprises an expansion part which produces expansion in the circumferential direction of the tube, and downstream of the mandrel the exertion of an axial pulling force on the tube, in the course of which the tube encounters a resistance which counteracts the movement of the tube over the mandrel. In the context of the present invention the term tube also relates to hose-type products.
2. Description of the Prior Art
The object of biaxially orienting the plastic material of a tube, also known as biaxially drawing a tube, is to improve the properties of the tube through orientation of the molecules of the plastic material of the tube in two mutually perpendicular directions. In the case of a certain generally known method the biaxial orientation is produced by forcing the tube over a mandrel at a suitable temperature, by exerting an axial pulling force on the tube downstream of the mandrel. Forcing the tube over the mandrel can also be assisted by additionally exerting a pushing force on the tube upstream of the mandrel, in the direction of the mandrel. This leads to a reduction in the necessary pulling force and makes a greater degree of drawing possible. The mandrel comprises an expansion part which produces the increase in the dimensions in the circumferential direction of the tube. It is essentially this which determines the orientation of the plastic material in the circumferential direction. The axial pulling force essentially determines the orientation in the axial direction. The biaxial orientation obtained is fixed (frozen) by cooling the tube.
A method of the type described in the preamble is already known from DE 2,357,210 (Petzetakis). This publication describes how a round tube with a relatively thick wall is manufactured in a known manner by means of an extruder. Viewed in the direction of movement of the tube, a solid, undeformable mandrel with a tapered expansion part is situated downstream of the extruder. The tube is forced over the mandrel at an orientation temperature which is suitable for the plastic material concerned, through an axial pulling force being exerted on the tube downstream of the mandrel. The diameter of the tube consequently increases, and the wall thickness decreases.
In the path between the extruder and the conical expansion part of the mandrel the extruded tube passes through means whose purpose is to produce the most homogeneous tube possible before the expansion in the circumferential direction occurs. This means that an effort is made to obtain a uniform wall thickness in the circumferential direction of the tube wall, inter alia by carrying out a calibration of the external diameter. An effort is also made to bring the plastic material of the tube wall to a uniform temperature, which temperature is the most suitable temperature for the envisaged biaxial orientation in the case in question.
It is found in practice that the manufacture of biaxially oriented tubing by the known method is not sufficiently controllable. This makes the known method unsuitable for use as a continuous process on an industrial scale, in particular because it is not found possible to obtain biaxially oriented tubing of an acceptable quality. In particular, it is found that with the known method it is not possible to obtain a tube with a sufficiently uniform wall thickness and biaxial orientation. An example of the unsatisfactory controllability of the process is that with the known method often a tube with a thickened part in the tube wall extending in the lengthwise direction of the tube is obtained.